1. Field of the Invention
The present invention relates to a webbing take-up device, and in particular to a motor retractor that can take up a webbing by a motor causing a take-up shaft to rotate.
2. Description of the Related Art
Seat belt devices that restrain passengers seated in seats with a long band-like webbing are attached to vehicles such as passenger cars, and a retractor (webbing take-up device) that stores the webbing in a state where the webbing is taken up such that it can be pulled out is disposed in so-called 3-point seat belt devices.
The retractor is disposed with a take-up shaft, to which the longitudinal-direction base end side of the webbing is fastened and which takes up the webbing from its base end side by rotating, and a biasing member such as a spiral spring, which biases the take-up shaft in the take-up direction of the webbing. Additionally, in a seat belt device disposed with this retractor, the take-up shaft is biased in the take-up direction of the webbing by the biasing force of the biasing member when a passenger wears the webbing, whereby the webbing from which slack has been removed restrains the passenger. Further, the webbing is taken up on the take-up shaft by the biasing force of the biasing member when the passenger lifts the worn state of the webbing.
Here, when the biasing force of the biasing member is small, the webbing is not able to be completely taken up and becomes slack, which causes the worsened appearance of the webbing when the webbing is not in use. On the other hand, when the biasing force of the biasing member is large, this imparts a feeling of tightness to the passenger wearing the webbing.
For this reason, a motor retractor is being considered which is disposed with a mechanism (so-called “storage assist mechanism”) that drives the take-up shaft by the drive force of a motor in order to reduce the biasing force of the biasing member and alleviate (control) the feeling of tightness imparted to the passenger and to assist the lowering of the webbing take-up force on the take-up shaft resulting from this reduction of the biasing force (e.g., see Japanese Patent Application Publication No. 2004-244011).
A motor retractor disposed with such a storage assist mechanism is usually disposed with a control circuit that controls the supply current to the motor. The control circuit is configured to supply current to the motor when it is detected that the worn state of the webbing has been lifted and to cut off the supply of current to the motor when it is detected that the webbing is completely stored and stall current is flowing to the motor, for example.
Incidentally, in a motor retractor of this configuration, it is necessary to stop the motor when a foreign object becomes entangled in the webbing while the webbing is being taken up. In this case, for example, it is conceivable to detect the stall current occurring in the motor by the entanglement of the foreign object and stop the motor. It is preferable for the motor retractor to be configured such that the motor is stopped by the detection of the stall current in this manner, because the supply current can be controlled by an existing control circuit.
However, because the current value of the current supplied to the motor is set to be somewhat large on the basis of an appropriate storing speed of the webbing, the current value of the detectable stall current also ends up becoming inevitably large. When the current value of the detectable stall current is large in this manner, the motor cannot be stopped in a low torque state when a foreign object has become entangled in the webbing, and for this reason, a measure to solve this has been sought after.